The present invention is a method and apparatus to improve the performance and efficiency of ultrasonic sensors which are used for such important tasks as non-invasive medical imaging and non-destructive industrial testing, such as checking the safety of nuclear power plants.
Ultrasonic imaging sensors act as both transmitters and receivers of ultrasonic energy. The sensor first acts as a transmitter; emitting ultrasonic energy in a train of high frequency pulses, typically in the range of 2 to 10 Mhz. Then the transmitter is turned off and the sensor acts as a receiver, which listens for returned echoes at the transmitted frequency.
A high performance ultrasonic sensor must be sensitive, accurate and have a low level of spurious acoustic responses including noise when excited by a short drive pulse. An acoustic pulse obtained from a short drive pulse gives good axial resolution, but it has a very broad frequency spectrum. The broad frequency spectrum excites spurious acoustic resonances called modes within the sensor. These modes tend to degrade its frequency response and consequently its ability to accurately differentiate closely spaced targets or impedance discontinuities when imaging parts of the human body. A system consisting of an imaging sensor and the associated electronic circuitry must be capable of transmitting the broadest range of frequencies by eliminating or suppressing the spurious modes, so as to enhance its sensitivity to detect the desired targets.
Existing ultrasonic sensors are designed to have a particular resonant frequency, which is the frequency at which desired mechanical motion is maximized. At that resonant frequency, the sensor elements are intended to vibrate along a preferred direction of sound propagation. However, driving the sensor at the desired resonant frequency will cause some of the energy to be coupled orthogonally to the desired motion. Such orthogonal motions represent sources of spurious modes in an imaging transducer. For example, if the sensor is in the form of a flat plate, the desired motion, or resonance, is in its thickness dimension. Undesired motions, called dilatational resonance modes, or dilatational modes, occur along the length and width of the plate.
The frequency of the modes is inversely proportional to these dimensions. Consequently, if the width is close to the thickness dimension, the spurious dilatational mode will fall close to the pass band of the thickness mode. This is shown in FIG. 2 for the element shown in FIG. 1.
Depending on whether the width is greater than, or less than, the thickness, the spurious modes will fall below or above the main resonance. The length dimension is usually much greater than the width, so that the spurious length mode is of very low frequency.
Both situations exist in medical sensors. In an annular array sensor, whose aspect ratio is less than unity, the spurious modes are below the desired response, as shown in FIGS. 2 through 5. In linear arrays, where the aspect ratio is greater than unity, the spurious modes are above the desired response.
In either case, these modes will lengthen the acoustic pulse transmitted into the body, and degrade the axial resolution. Accurate axial resolution translates into an ability to see fine details of tissue structure, and provides the physician a powerful diagnostic tool.
A dilatational mode along the width is shown in FIG. 1. These undesired motions cause energy to be radiated into, and received from, directions other than that intended. The result is that returns are received from undesired directions, and these unwanted returns constitute noise. This unwanted noise is mixed in with the desired signals, thereby degrading the received signals.
The development of an effective method of suppressing the undesirable dilatational modes would constitute a major technological advance in the technology of ultrasonic imaging. The improved performance that would result from such an innovation would substantially improve the performance of ultrasonic imaging equipment used for medical imaging and for important industrial applications.